Anion gap and hypoalbuminemia

Objectives To show how hypoalbuminemia lowers the anion gap, which can mask a significant gap acidosis; and to derive a correction factor for it. Design Observational study. Setting Intensive care unit in a university-affiliated hospital. Subjects Nine normal subjects and 152 critically ill patients (265 measurements). Interventions None. Measurements and Main Results Arterial blood samples analyzed for pH, PCO (2), and concentrations of plasma electrolytes and proteins. Marked hypoalbuminemia was common among the critically ill patients: 49% of them had serum albumin concentration of <20 g/L. Each g/L decrease in serum albumin caused the observed anion gap to underestimate the total concentration of gap anions by 0.25 mEq/L (r2 = .94). Conclusions The observed anion gap can be adjusted for the effect of abnormal serum albumin concentrations as follows: adjusted anion gap = observed anion gap + 0.25 x ([normal albumin]-[observed albumin]), where albumin concentrations are in g/L; if given in g/dL, the factor is 2.5. This adjustment returns the anion gap to the familiar scale of values that apply when albumin concentration is normal. (Crit Care Med 1998; 26:1807-1810)